Catalytic depolymerization method for nylon 6 extraction oligomer

Abstract

The invention relates to a catalytic depolymerization method of a nylon 6 extraction oligomer, and relates to the technical field of polymer depolymerization, a nylon 6 extraction oligomer concentrated solution and a depolymerization catalyst are mixed, heated and subjected to ultrasonic treatment, and a premix is obtained; and performing staged gradient depolymerization on the premix in an inert gas atmosphere to obtain a depolymerization product. The staged gradient depolymerization of the nylon 6 extracted oligomer is realized by controlling the reaction pressure and the reaction temperature in the depolymerization reaction system to be subjected to staged gradient change along with the reaction time, the depolymerization process is simple to operate, other raw materials do not need to be added, the depolymerization rate of the nylon 6 extracted oligomer is increased, and the production cost is reduced. The technical problems of poor catalytic depolymerization effect, tedious operation steps and the like in the conventional catalytic depolymerization process of the nylon 6 extracted oligomer concentrated solution are effectively solved.

Description

Technical Field

[0001] The present application relates to the technical field of polymer depolymerization, and in particular to a method for catalytic depolymerization of nylon 6 extraction oligomers. Background Art

[0002] Polyamide 6, commonly known as nylon 6 (PA6), is a crystalline polymer with good comprehensive properties and is one of the most widely used types of engineering plastics. Nylon 6 chips, also known as nylon 6 chips, are the second largest synthetic fiber and play an important role in industrial use.

[0003] At present, caprolactam hydrolysis polymerization is the most widely used nylon 6 polymerization production process. Since the reaction is a reversible equilibrium reaction, the residual 10% of caprolactam monomer (about 8%) and oligomer (about 2%) in the equilibrium polymer are collectively referred to as extractables. The oligomers mainly include linear oligomers and cyclic oligomers, of which the content of cyclic oligomers is about 3wt% to 4wt%, and the amount of cyclic dimers accounts for about 70wt% of the total content of cyclic oligomers. In industrial production, the extractable content in nylon 6 chips has a relatively large impact on chip spinning. Generally, the mass fraction of extractables in the chips is required to be less than 0.6%, while high-speed spinning requires this index to be less than 0.5%.

[0004] In order to remove low molecular substances in polyamide 6 slices, hot water countercurrent extraction method and extraction water evaporation concentration recovery technology are often used in industry to obtain nylon 6 extracted oligomer concentrate, and then the nylon 6 extracted oligomer concentrate is directly returned or cracked and returned to the polymerization reaction section to directly participate in the reaction, so as to realize the recycling of caprolactam raw materials. However, the catalytic depolymerization effect of nylon 6 extracted oligomer concentrate in the process of producing nylon 6 by hydrolysis polymerization of caprolactam is poor, resulting in excessive dimer content in nylon 6 slices, affecting product quality, and even blocking pipelines during precipitation. Moreover, the catalytic depolymerization process of nylon 6 extracted oligomer concentrate in the prior art has the problems of complicated operation steps, high energy consumption, and low quality of caprolactam monomer recovery. Summary of the invention

[0005] The present application provides a method for catalytic depolymerization of nylon 6 extract oligomers to solve the technical problems of poor catalytic depolymerization effect and complicated operation steps in the existing catalytic depolymerization process of nylon 6 extract oligomer concentrate, and further effectively avoids a series of problems caused by excessive content of cyclic dimers and the like in the nylon 6 extract oligomer concentrate.

[0006] The present application provides a catalytic depolymerization method for nylon 6 extract oligomers, the catalytic depolymerization method comprising the following steps:

[0007] The nylon 6 oligomer extract concentrate and the depolymerization catalyst are mixed, heated and ultrasonically treated to obtain a premix;

[0008] Under an inert gas atmosphere, the premix is ​​subjected to a stepwise gradient depolymerization to obtain a depolymerization product;

[0009] Purifying the depolymerization product to obtain caprolactam;

[0010] The specific process of the step-by-step gradient depolymerization includes: the reaction pressure and the reaction temperature in the depolymerization reaction system are both subjected to step-by-step gradient changes along with the reaction time until the catalytic depolymerization reaction of the nylon 6 extraction oligomer is completed.

[0011] Furthermore, the weight percentage of water in the nylon 6 oligomer extract concentrate is 5wt% to 150wt%.

[0012] Furthermore, based on the weight of the nylon 6 extracted oligomers, the added amount of the depolymerization catalyst accounts for 0.1wt% to 5wt% of the weight of the nylon 6 extracted oligomers.

[0013] Furthermore, the depolymerization catalyst is a solid acid catalyst or a liquid acid catalyst, and the solid acid catalyst includes at least one of a solid phosphoric acid catalyst, a solid sulfuric acid catalyst and a solid superacid catalyst.

[0014] Furthermore, the preparation method of the solid phosphoric acid catalyst comprises:

[0015] The solid phosphoric acid catalyst is obtained by mixing pyrophosphoric acid and phosphorus pentoxide in a weight ratio of 1:(1.5-3.0), and then adding diatomaceous earth for heat treatment.

[0016] Furthermore, the step-wise gradient depolymerization operation mode includes an intermittent catalytic depolymerization operation mode or a continuous catalytic depolymerization operation mode.

[0017] Furthermore, when the stepwise gradient depolymerization is performed by an intermittent catalytic depolymerization operation mode, the step of performing stepwise gradient depolymerization on the premix under an inert gas atmosphere to obtain a depolymerization product comprises:

[0018] Under an inert gas atmosphere, the premix is ​​added into a depolymerization reaction device at one time, and then a first depolymerization reaction stage, a second depolymerization reaction stage, a third depolymerization reaction stage and a fourth depolymerization reaction stage are sequentially performed to obtain a depolymerization product;

[0019] The working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C to 220°C and the reaction pressure to 1.6MPa to 3.0MPa within 10min to 60min, and then maintaining for 10min to 60min;

[0020] The working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C to 250°C and the reaction pressure to 2.0MPa to 3.4MPa within 10min to 30min, and then maintaining for 10min to 60min;

[0021] The working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260°C to 280°C and the reaction pressure to 2.4MPa to 3.6MPa within 10min to 30min, and then maintaining for 30min to 120min;

[0022] The working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. to 220° C. and the reaction pressure to 1.6 MPa to 3.0 MPa within 10 to 45 minutes, and then maintaining them for 30 to 300 minutes.

[0023] Further, the working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C to 210°C and the reaction pressure to 1.6MPa to 2.6MPa within 10min to 40min, and then maintaining for 10min to 30min;

[0024] The working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C to 240°C and the reaction pressure to 2.0MPa to 2.8MPa within 10min to 20min, and then maintaining for 10min to 30min;

[0025] The working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260°C to 270°C and the reaction pressure to 2.4MPa to 3.0MPa within 15min to 25min, and then maintaining for 30min to 60min;

[0026] The working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. to 210° C. and the reaction pressure to 1.6 MPa to 2.6 MPa within 10 to 20 minutes, and then maintaining them for 30 to 150 minutes.

[0027] Furthermore, when the stepwise gradient depolymerization is performed in a continuous catalytic depolymerization operation mode, the step of performing stepwise gradient depolymerization on the premix under an inert gas atmosphere to obtain a depolymerization product comprises:

[0028] Under an inert gas atmosphere, continuously conveying the premix to a multi-stage series depolymerization reaction device for depolymerization to obtain a depolymerization product;

[0029] The multi-stage series depolymerization reaction device comprises at least three first depolymerization reactors, a second depolymerization reactor and a third depolymerization reactor which are sequentially arranged in series;

[0030] The operating parameters of the first depolymerization reactor include: temperature of 200°C to 240°C, pressure of 1.6MPa to 3.4MPa, and residence time of 10min to 90min;

[0031] The operating parameters of the second depolymerization reactor include: temperature of 260°C to 280°C, pressure of 2.4MPa to 3.6MPa, and residence time of 10min to 90min;

[0032] The operating parameters of the third depolymerization reactor include: temperature of 200° C. to 240° C., pressure of 1.6 MPa to 3.4 MPa, and residence time of 20 min to 300 min.

[0033] Further, the operating parameters of the first depolymerization reactor include: temperature of 230°C to 240°C, pressure of 2.0MPa to 3.0MPa, and residence time of 30min to 60min;

[0034] The operating parameters of the second depolymerization reactor include: temperature of 265°C to 275°C, pressure of 3.0MPa to 3.2MPa, and residence time of 50min to 80min;

[0035] The operating parameters of the third depolymerization reactor include: temperature of 210° C. to 230° C., pressure of 2.0 MPa to 3.0 MPa, and residence time of 50 min to 200 min.

[0036] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0037] The embodiment of the present application provides a catalytic depolymerization method for nylon 6 extracted oligomers, which uses an ultrasonic mixing-acid catalytic depolymerization process to depolymerize oligomers in the extracted concentrated solution obtained in the nylon 6 preparation process, and controls the reaction pressure and reaction temperature in the depolymerization reaction system to perform stage-by-stage gradient changes with the reaction time, thereby realizing stage-by-stage gradient depolymerization of nylon 6 extracted oligomers. The depolymerization process is simple to operate, and no other raw materials need to be added. The depolymerization rate of nylon 6 extracted oligomers (including cyclic dimers) is improved, and the technical problems of poor catalytic depolymerization effect and complicated operation steps in the existing catalytic depolymerization process of nylon 6 extracted oligomer concentrated solution are effectively solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0039] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0040] Figure 1 A schematic flow chart of a catalytic depolymerization method of nylon 6 extraction oligomers provided in an embodiment of the present application;

[0041] Figure 2 The process flow chart of preparing nylon 6 by hydrolysis and polymerization of caprolactam in the prior art is adopted in this application;

[0042] Figure 3 Schematic diagram of the structure of the nylon 6 extraction oligomer depolymerization reaction device in the embodiment of the present application;

[0043] Among them, 1-high pressure reactor; 2-oil bath; 3-magnetic stirrer; 4-temperature controller; 5-air release valve. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.

[0045] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0046] The present application provides a method for catalytic depolymerization of nylon 6 extract oligomers, such as Figure 1 As shown, the catalytic depolymerization method comprises the following steps:

[0047] The nylon 6 oligomer extract concentrate and the depolymerization catalyst are mixed, heated and ultrasonically treated to obtain a premix;

[0048] Under an inert gas atmosphere, the premix is ​​subjected to a stepwise gradient depolymerization to obtain a depolymerization product;

[0049] Purifying the depolymerization product to obtain caprolactam;

[0050] The specific process of the step-by-step gradient depolymerization includes: the reaction pressure and the reaction temperature in the depolymerization reaction system are both subjected to step-by-step gradient changes along with the reaction time until the catalytic depolymerization reaction of the nylon 6 extraction oligomer is completed.

[0051] The embodiment of the present application provides a catalytic depolymerization method for nylon 6 extracted oligomers. The method uses an ultrasonic mixing-acid catalytic depolymerization process to depolymerize the oligomers in the extracted concentrated solution obtained in the nylon 6 preparation process, and controls the reaction pressure and reaction temperature in the depolymerization reaction system to change in a staged gradient with the reaction time, so as to achieve a staged gradient depolymerization of the nylon 6 extracted oligomers. The depolymerization process is simple to operate, and no other raw materials need to be added. The depolymerization rate of the nylon 6 extracted oligomers is improved (the depolymerization rate of the cyclic dimer is ≥80%), and the technical problems such as poor catalytic depolymerization effect and complicated operation steps in the existing catalytic depolymerization process of the nylon 6 extracted oligomer concentrated solution are effectively solved.

[0052] The nylon 6 extracted oligomers described in the present application specifically refer to those obtained by extracting nylon 6 chips in the existing nylon 6 polymerization production process (caprolactam hydrolysis polymerization process), which mainly include linear oligomers and cyclic oligomers, wherein the content of cyclic oligomers is about 3wt% to 4wt%, and the amount of cyclic dimers accounts for about 70wt% of the total content of cyclic oligomers.

[0053] The nylon 6 oligomer extract concentrate described in this application specifically refers to the concentrate obtained by evaporating and concentrating the extract water obtained by extracting polyamide 6 slices in the existing nylon 6 polymerization production process (caprolactam hydrolysis polymerization process), which mainly includes caprolactam monomers and oligomers (i.e., the nylon 6 oligomer extract); for example, the polyamide 6 slice core can be extracted by the conventional hot water countercurrent extraction method in the industry, and then the extract water can be evaporated, concentrated and recovered to obtain the nylon 6 oligomer extract concentrate. In some specific embodiments, such as Figure 2 As shown, this application adopts the process flow of preparing nylon 6 in the prior art, through the hydrolysis polymerization of caprolactam, preheating and entering the polymerization section, the polymerization product nylon 6 contains about 10% extractables. The extractables are mainly unreacted caprolactam, cyclic oligomers and a small amount of linear oligomers; in the extraction section, the extractables are extracted by hot water to form an extract, and the extractable content in nylon 6 is reduced to less than 0.6% and then enters the drying section; the extract is evaporated by triple effect in the evaporation section to remove most of the water to form a nylon 6 extracted oligomer concentrate.

[0054] The purpose of mixing, heating and ultrasonically treating the nylon 6 oligomer extract concentrate and the depolymerization catalyst in this application is to strengthen the dispersion and mixing of the nylon 6 oligomer extract concentrate and the depolymerization catalyst to improve the subsequent catalytic depolymerization efficiency and uniformity. In some specific embodiments, the nylon 6 oligomer extract concentrate and the depolymerization catalyst are mixed, heated and ultrasonically treated to obtain a premix, including: heating and stirring the nylon 6 oligomer extract concentrate and the depolymerization catalyst in a water bath, and ultrasonically treating the nylon 6 oligomer extract concentrate and the depolymerization catalyst using an ultrasonic vibration assembly composed of a conventional existing ultrasonic vibration system and an existing ultrasonic generator device in the art, and the working parameters of the ultrasonic treatment include: an ultrasonic power of 1.0 to 2.0 kW, an ultrasonic frequency of 40 to 60 kHz, and an ultrasonic duration of 0.5 to 1.5 hours, thereby fully achieving the dispersion and mixing of the nylon 6 oligomer extract concentrate and the depolymerization catalyst.

[0055] The purification post-treatment of the depolymerization product described in the present application can adopt conventional purification operations in the art, such as water washing, filtration, recrystallization, resin adsorption depolymerization, etc. In some specific embodiments, the depolymerization product can be subjected to water washing-filtration-crystallization-drying post-treatment operations in sequence to obtain a white powdery caprolactam depolymerization product (the purity of caprolactam can reach 99.5%), which is simple to operate and low in cost. Subsequently, the obtained white powdery caprolactam depolymerization product is treated with Soxhlet extraction and dissolved in methanol for quantitative analysis, and the depolymerization rate is calculated by detecting the cyclic dimer content using HPLC.

[0056] It should be noted that the above-mentioned Soxhlet extraction and vacuum distillation operations in this application, unless otherwise specified or limited, can be performed using conventional existing technologies in the art, and this application document will not describe them one by one.

[0057] As an implementation method of the present application, the weight percentage of water in the nylon 6 oligomer extract concentrate is 5wt% to 150wt%.

[0058] The weight percentage of water in the nylon 6 extracted oligomer concentrate described in the present application is 5wt% to 150wt%, preferably 15wt% to 40wt%, specifically 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt%, 21wt%, 22wt%, 23wt%, 24wt%, 26wt%, 27wt%, 28wt%, 29wt%, 30wt%, 31wt%, 32wt%, 33wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, 40wt% and the like.

[0059] As an implementation method of an example of the present application, based on the weight of the nylon 6 extracted oligomers, the added amount of the depolymerization catalyst accounts for 0.1 wt% to 5 wt% of the weight of the nylon 6 extracted oligomers.

[0060] Based on the weight of the nylon 6 extract oligomer, the application controls the addition amount of the depolymerization catalyst to account for 0.1wt% to 5wt% of the weight of the nylon 6 extract oligomer. The addition of the depolymerization catalyst can promote the depolymerization reaction, control the degree of depolymerization, optimize the reaction conditions and improve the product quality in the production of nylon 6 extract oligomer. Reasonable selection of the addition amount of the depolymerization catalyst can achieve the desired depolymerization effect and meet the production requirements. In some specific embodiments, the amount of the depolymerization catalyst added may be 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.2wt%, 1.3wt%, 1.4wt%, 1.5wt%, 1.6wt%, 1.7wt%, 1.8wt%, 1.9wt%, 2.0wt%, 2.1wt%, 2.2wt%, 2.3wt%, 2.4wt%, 2.5wt%, 2.6wt%, 2.7wt%, 2.8wt%, 2.9wt%, 3.0wt%, 3.1wt%, 3.2wt%, 3.3wt%, 3.4wt%, 3.5wt%, 3.6wt%, 3.7wt%, 3.8wt%, 3.9wt%, 4.0wt%, 4.1wt%, 4.2wt%, 4.3wt%, 4.4wt%, 4.5wt%, 4.6wt%, 4.7wt%, 4.8wt%, 4.9wt%, 5. .4wt%, 2.5wt%, 2.6wt%, 2.7wt%, 2.8wt%, 2.9wt%, 3.0wt%, 3.1wt%, 3.2wt%, 3.3wt%, 3.4wt%, 3.5wt%, 3.6wt%, 3.7wt%, 3.8wt%, 3.9wt%, 4.0wt%, 4.1wt%, 4.2wt%, 4.3wt%, 4.4wt%, 4.5wt%, 4.6wt%, 4.7wt%, 4.8wt%, 4.9wt%, 5.0wt% etc.; preferably 3.0wt% to 5.0wt%.

[0061] As an implementation method of an example of the present application, the depolymerization catalyst is a solid acid catalyst or a liquid acid catalyst, and the solid acid catalyst includes at least one of a solid phosphoric acid catalyst, a solid sulfuric acid catalyst and a solid superacid catalyst.

[0062] The present application selects a solid acid catalyst or a liquid acid catalyst as a depolymerization catalyst, which can improve the efficiency of catalytic cracking.

[0063] In some specific embodiments, the liquid acid catalyst may include a strong acid that is soluble in water and completely ionized and / or a weak acid that is soluble in water and partially ionized; for example, the strong acid may be hydrochloric acid, sulfuric acid, nitric acid, hydrobromic acid, etc.; the weak acid may be acetic acid, phosphoric acid, hydrofluoric acid, hydrocyanic acid, etc.

[0064] In some specific embodiments, the carrier of the solid acid catalyst may be an existing carrier such as a molecular sieve carrier, a functional polymer carrier, a carbon material carrier, etc.

[0065] The superacid described in this application is also called superacid, which refers to an acid with stronger acidity than pure sulfuric acid, such as fluoroantimony sulfonic acid.

[0066] As an implementation method of the present application, the preparation method of the solid phosphoric acid catalyst includes:

[0067] The solid phosphoric acid catalyst is obtained by mixing pyrophosphoric acid and phosphorus pentoxide in a weight ratio of 1:(1.5-3.0), and then adding diatomaceous earth for heat treatment.

[0068] The catalytic depolymerization effect of the solid phosphoric acid catalyst obtained by the above-mentioned preparation method is better in the present application. In some specific embodiments, the preferred weight ratio of pyrophosphoric acid and phosphorus pentoxide is 1:2; the heat treatment process includes: preheating the mixture of pyrophosphoric acid, phosphorus pentoxide and diatomaceous earth to 150-200°C, then treating it at a medium temperature of 160-240°C for 2-3h, and finally calcining it at a high temperature of 500-600°C for 1-2h, and then treating it with water vapor. The purpose of the above-mentioned staged heat treatment selected in the present application is that different treatment temperatures will generate silicophosphates with different crystalline phases, which will also affect the acid strength distribution of the catalyst.

[0069] As an implementation mode of the examples of the present application, the operation mode of the staged gradient depolymerization includes an intermittent catalytic depolymerization operation mode or a continuous catalytic depolymerization operation mode.

[0070] The present application can adopt an intermittent catalytic depolymerization operation mode or a continuous catalytic depolymerization operation mode to control the reaction pressure and reaction temperature in the depolymerization reaction system to change in stages with the reaction time, thereby realizing staged gradient depolymerization.

[0071] As an implementation mode of the present application, when the intermittent catalytic depolymerization operation mode is used to perform the stepwise gradient depolymerization, the step of performing the stepwise gradient depolymerization of the premix under an inert gas atmosphere to obtain the depolymerization product includes:

[0072] Under an inert gas atmosphere, the premix is ​​added into a depolymerization reaction device at one time, and then a first depolymerization reaction stage, a second depolymerization reaction stage, a third depolymerization reaction stage and a fourth depolymerization reaction stage are sequentially performed to obtain a depolymerization product;

[0073] The working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C to 220°C and the reaction pressure to 1.6MPa to 3.0MPa within 10min to 60min, and then maintaining for 10min to 60min;

[0074] The working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C to 250°C and the reaction pressure to 2.0MPa to 3.4MPa within 10min to 30min, and then maintaining for 10min to 60min;

[0075] The working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260°C to 280°C and the reaction pressure to 2.4MPa to 3.6MPa within 10min to 30min, and then maintaining for 30min to 120min;

[0076] The working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. to 220° C. and the reaction pressure to 1.6 MPa to 3.0 MPa within 10 to 45 minutes, and then maintaining them for 30 to 300 minutes.

[0077] The applicant has found through a large number of experiments that when the step-by-step gradient depolymerization is performed in an intermittent catalytic depolymerization operation mode, the catalytic depolymerization effect of nylon 6 extract oligomers is better by using the step-by-step gradient depolymerization with the above-mentioned specific parameters. The specific principles of using the above-mentioned specific parameters include: the main purpose of optimizing the process parameters is to reduce the content of cyclic oligomers and to increase the yield and purity of the target product. Through a series of experiments and simulation calculations, the control method of temperature and pressure at different depolymerization stages can be obtained.

[0078] Preferably, the working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C to 210°C and the reaction pressure to 1.6MPa to 2.6MPa within 10min to 40min, and then maintaining them for 10min to 30min.

[0079] Preferably, the working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C to 240°C and the reaction pressure to 2.0MPa to 2.8MPa within 10min to 20min, and then maintaining them for 10min to 30min.

[0080] Preferably, the working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260° C. to 270° C. and the reaction pressure to 2.4 MPa to 3.0 MPa within 10 to 20 minutes, and then maintaining them for 30 to 60 minutes.

[0081] Preferably, the working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. to 210° C. and the reaction pressure to 1.6 MPa to 2.6 MPa within 10 to 20 minutes, and then maintaining them for 30 to 150 minutes.

[0082] The present application can use the existing cracking reaction device to realize the intermittent catalytic depolymerization operation, and the temperature and pressure are quickly lowered after the reaction is completed, and the product is discharged from the depolymerization reactor. In some specific embodiments, the following can be specifically used: Figure 3 The cracking reaction device shown completes the depolymerization reaction of nylon 6 extraction oligomers, and the cracking reaction device includes a high-pressure reactor 1, an oil bath pot 2, a magnetic stirrer 3, a temperature controller 4 and a vent valve 5. The high-pressure reactor 1 is placed in the oil bath pot 2 for oil bath insulation, and the temperature in the high-pressure reactor 1 is controlled by the temperature controller 4 to maintain the reaction temperature. Furthermore, the cracking reaction device also includes components such as a pressure gauge to achieve pressure control in the reaction system.

[0083] It should be noted that the existing cracking reaction device used in the present application, if there is no special limitation or requirement, can adopt existing equipment such as conventional polymerization reactors in the art and be connected according to existing process design methods, which will not be described in detail here.

[0084] As an implementation mode of the present application, when the step-wise gradient depolymerization is performed by a continuous catalytic depolymerization operation mode, the step of performing step-wise gradient depolymerization on the premix under an inert gas atmosphere to obtain a depolymerization product includes:

[0085] Under an inert gas atmosphere, continuously conveying the premix to a multi-stage series depolymerization reaction device for depolymerization to obtain a depolymerization product;

[0086] The multi-stage series depolymerization reaction device comprises at least three first depolymerization reactors, a second depolymerization reactor and a third depolymerization reactor which are sequentially arranged in series;

[0087] The operating parameters of the first depolymerization reactor include: temperature of 200°C to 240°C, pressure of 1.6MPa to 3.4MPa, and residence time of 10min to 90min;

[0088] The operating parameters of the second depolymerization reactor include: temperature of 260°C to 280°C, pressure of 2.4MPa to 3.6MPa, and residence time of 10min to 90min;

[0089] The operating parameters of the third depolymerization reactor include: temperature of 200° C. to 240° C., pressure of 1.6 MPa to 3.4 MPa, and residence time of 20 min to 300 min.

[0090] The applicant has found through a large number of experiments that when the step-by-step gradient depolymerization is carried out in a continuous catalytic depolymerization operation mode, the catalytic depolymerization effect of nylon 6 extract oligomers is better by using the step-by-step gradient depolymerization with the above-mentioned specific parameters. The specific principles of using the above-mentioned specific parameters include: the continuous depolymerization operation has the advantages of higher production efficiency and stable product quality compared with the intermittent depolymerization operation. The staged depolymerization process can better control the reaction process, improve product quality and production flexibility. .

[0091] Preferably, the operating parameters of the first depolymerization reactor include: temperature of 230° C. to 240° C., pressure of 2.0 MPa to 3.0 MPa, and residence time of 30 min to 60 min.

[0092] Preferably, the operating parameters of the second depolymerization reactor include: temperature of 265° C. to 275° C., pressure of 3.0 MPa to 3.2 MPa, and residence time of 50 min to 80 min.

[0093] Preferably, the operating parameters of the third depolymerization reactor include: temperature of 210° C. to 230° C., pressure of 2.0 MPa to 3.0 MPa, and residence time of 50 min to 200 min.

[0094] The present application can adopt the existing multi-stage series depolymerization reaction system to realize continuous catalytic depolymerization operation, including a first depolymerization reactor, a second depolymerization reactor and a third depolymerization reactor, the depolymerization reactor is a depolymerization reactor and / or a tubular reactor, a static mixer is arranged in the tubular reactor to ensure that the radial temperature field and concentration field in the tube are uniform when the material flows through the tubular reactor, so that the material flow shape in the tubular reactor is close to the plug flow, and after the reaction material flows through each reactor in sequence, the final product is discharged from the outlet of the last stage depolymerization reactor; the temperature and pressure of each stage depolymerization reactor do not change with time, and are in steady-state operation; a static mixer can be arranged in the tubular reactor.

[0095] It should be noted that the multi-stage series depolymerization reaction system used in the present application, if there are no special limitations and requirements, can adopt existing equipment such as conventional polymerization reactors in the art and be connected according to existing process design methods, which will not be described in detail here.

[0096] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.

[0097] Example 1

[0098] This embodiment provides a method for catalytic depolymerization of nylon 6 extract oligomers, wherein the method adopts an intermittent catalytic depolymerization operation mode to perform the step-by-step gradient depolymerization, and comprises the following steps:

[0099] S1: preheating the reactor in an oil bath pot, the preheating temperature of the oil bath pot is 70° C. adding the nylon 6 extract concentrate obtained by the treated nylon 6 production process (i.e., the original nylon 6 extract concentrate) and the depolymerization catalyst into the preheated reactor and ultrasonically treating them to obtain a premix;

[0100] The working parameters of ultrasonic treatment included: ultrasonic power of 1.0 kW, ultrasonic frequency of 4 kHz, and ultrasonic duration of 1.0 h;

[0101] The mass percentage of water in the nylon 6 extract concentrate is 150wt%;

[0102] The amount of the depolymerization catalyst added is 5wt% of the mass percentage of the extract concentrate;

[0103] The depolymerization catalyst is a mixture of pyrophosphoric acid and phosphorus pentoxide in a mass ratio of 1:2, followed by the addition of diatomaceous earth (the specific amount of diatomaceous earth added is 10wt%), and then heat-treated to obtain the depolymerization catalyst. The heat treatment includes preheating the mixture of pyrophosphoric acid, phosphorus pentoxide and diatomaceous earth to 180°C for 1.0h, then treating it at a medium temperature of 200°C for 2.5h, and finally calcining it at a high temperature of 550°C for 1.5h, and steam-treating (exposing the mixture to a saturated water vapor environment to adjust the reaction temperature, promote the activity of the catalyst and pre-activate the catalyst) to obtain the depolymerization catalyst;

[0104] S2: catalytic depolymerization, the reactor is subjected to gradient temperature and pressure increase, and the temperature and pressure are kept for a period of time each time they reach the specified temperature and pressure; the specific process is as follows:

[0105] Under an inert gas atmosphere, the premix is ​​added into a depolymerization reaction device at one time, and then the first depolymerization reaction stage, the second depolymerization reaction stage, the third depolymerization reaction stage and the fourth depolymerization reaction stage are sequentially performed to obtain a depolymerization product; wherein the heating rate of each stage is: dT / dt≈17.5°C / min, and the pressure increase rate is: dP / dt≈159.9kPa / min;

[0106] The working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C and the reaction pressure to 1.6 MPa within 10 minutes, and then maintaining for 10 minutes;

[0107] The working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C and the reaction pressure to 2.0 MPa within 10 minutes, and then maintaining for 10 minutes;

[0108] The working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260° C. and the reaction pressure to 2.4 MPa within 10 minutes, and then maintaining them for 30 minutes;

[0109] The working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. and the reaction pressure to 1.6 MPa within 10 minutes, and then maintaining them for 60 minutes;

[0110] S3: After the post-treatment reaction is completed, the reactor is quickly placed in ice water for 5 minutes, and the temperature and pressure in the reactor are reduced to room temperature and pressure, and then discharged through the discharge port. The depolymerization product is subjected to post-treatment operations such as washing-filtration-crystallization-drying to obtain a white powdery depolymerization product, which is then dissolved in methanol for quantitative analysis after Soxhlet extraction;

[0111] The specific treatment method of Soxhlet extraction treatment is as follows: weigh about 3g of the pyrolysis product powder, wrap it with quantitative filter paper and place it in the middle of the Soxhlet extractor, add about 150mL of methanol into the round-bottom flask, and heat and extract continuously for 12 hours. After the extraction is completed, all the liquid phases in the Soxhlet extractor are recovered and the volume is adjusted to a 250mL volumetric flask. Since the solution concentration is still higher than the liquid chromatography analysis range at this time, errors may occur. Therefore, it is still necessary to accurately transfer 2mL with a pipette and adjust the volume to a 25mL volumetric flask, and finally inject it into a 2mL liquid chromatography analysis sample bottle after filtration for analysis by high performance liquid chromatography.

[0112] Instrument: Waters high performance liquid chromatography (HPLC) analysis system, Breeze data processing system. Chromatographic column: Waters C18 liquid chromatography column. Analysis wavelength: 210 nm. Mobile phase: methanol (chromatographic grade), water (ultrapure water) in a gradient ratio.

[0113] In this Example 1, the obtained depolymerization product and the original nylon 6 extract concentrate were treated and then the cyclic dimer content was detected by HPLC to obtain the cleavage rate of the cyclic dimer.

[0114] In this embodiment, the cleavage rate of the cyclic dimer is specifically 91.8%.

[0115] Example 2-8 investigates the effect of water content in the nylon 6 oligomer extract concentrate on the depolymerization of nylon 6 oligomer extract.

[0116] Based on Example 1, Examples 2-8 only adjust the weight percentage of water in the obtained nylon 6 extract oligomer concentrate (the remaining steps and parameters are the same as Example 1). The weight percentage of water in the nylon 6 extract oligomer concentrate in Examples 2-8 is shown in Table 1.

[0117] Table 1 Example 2-8 Investigating the effect of water content in nylon 6 oligomer extract concentrate

[0118]

[0119] Examples 9-14 investigate the effect of the amount of depolymerization catalyst added on the depolymerization of nylon 6 extract oligomers.

[0120] On the basis of Example 1, Examples 9-14 only adjusted the amount of the depolymerization catalyst added (the remaining steps and parameters were the same as those in Example 1). Based on the weight of the nylon 6 extracted oligomers, the percentage of the amount of the depolymerization catalyst added in Examples 9-14 to the weight of the nylon 6 extracted oligomers is shown in Table 2.

[0121] Table 2 Effect of the amount of depolymerization catalyst added in Examples 9-14

[0122]

[0123] Examples 15-20 investigate the effect of the type of depolymerization catalyst on the depolymerization of nylon 6 extract oligomers.

[0124] On the basis of Example 1, Examples 15-20 only adjusted the type of depolymerization catalyst (the remaining steps and parameters were the same as Example 1). The types of catalysts in Examples 15-20 are shown in Table 3.

[0125] Table 3 Effect of Depolymerization Catalyst Types in Examples 15-20

[0126]

[0127] Compared with the depolymerization catalyst in Example 1, the preparation method of the depolymerization catalyst A is different only in that: the heat treatment process includes treating the mixture of pyrophosphoric acid, phosphorus pentoxide and diatomaceous earth at a medium temperature of 200° C. for 2.5 h, and finally calcining at a high temperature of 550° C. for 1.5 h (that is, there is no preheating step in the heat treatment process, and the other steps are the same); the other steps and parameters are the same.

[0128] Compared with the depolymerization catalyst in Example 1, the preparation method of the depolymerization catalyst B is different only in that: the heat treatment process includes preheating the mixture of pyrophosphoric acid, phosphorus pentoxide and diatomaceous earth at 180° C. for 1.0 h, and then high-temperature calcining at 550° C. for 4 h (that is, there is no medium-temperature treatment step in the heat treatment process and the high-temperature calcination time is increased, and the other steps are the same); the other steps and parameters are the same.

[0129] Compared with the depolymerization catalyst in Example 1, the preparation method of the depolymerization catalyst C is different only in that: the heat treatment process includes preheating the mixture of pyrophosphoric acid, phosphorus pentoxide and diatomaceous earth at 180°C for 1.0h, then treating it at a medium temperature of 200°C for 2.5h, and then calcining it at a high temperature of 700°C for 1.5h (that is, the high temperature calcination temperature is increased to 700°C during the heat treatment process, and the other steps are the same); the other steps and parameters are the same.

[0130] Examples 21-31 investigate the effects of catalytic cracking parameters at each stage on the depolymerization of nylon 6 extract oligomers.

[0131] On the basis of Example 1, Examples 21-31 only adjust the catalytic cracking parameters at each stage (the remaining steps and parameters are the same as those in Example 1). The catalytic cracking parameters at each stage in Examples 21-31 are shown in Table 4, and the cyclic dimer cracking rates in Examples 21-31 are shown in Table 5.

[0132] Table 4 Catalytic cracking parameters at each stage of Examples 21-31

[0133]

[0134]

[0135] Examples 32-36 use a continuous catalytic depolymerization operation mode to carry out the step-wise gradient depolymerization.

[0136] Examples 32-36 provide a method for catalytic depolymerization of nylon 6 extract oligomers, wherein the method adopts a continuous catalytic depolymerization operation mode to perform the step-wise gradient depolymerization, and comprises the following steps:

[0137] The reactor is placed in an oil bath for preheating, and the preheating temperature of the oil bath is 70° C. The nylon 6 extract concentrate obtained by the treated nylon 6 production process (i.e., the original nylon 6 extract concentrate) and the depolymerization catalyst are added to the preheated reactor and ultrasonically treated to obtain a premix;

[0138] The working parameters of ultrasonic treatment included: ultrasonic power of 1.0 kW, ultrasonic frequency of 4 kHz, and ultrasonic duration of 1.0 h;

[0139] The mass percentage of water in the nylon 6 extract concentrate is 150wt%;

[0140] The amount of the depolymerization catalyst added is 5wt% of the mass percentage of the extract concentrate;

[0141] The depolymerization catalyst is a mixture of pyrophosphoric acid and phosphorus pentoxide in a mass ratio of 1:2, followed by the addition of diatomaceous earth (the specific amount of diatomaceous earth added is 10wt%), and then heat-treated to obtain the depolymerization catalyst. The heat treatment includes preheating the mixture of pyrophosphoric acid, phosphorus pentoxide and diatomaceous earth to 180°C for 1.0h, then treating it at a medium temperature of 200°C for 2.5h, and finally calcining it at a high temperature of 550°C for 1.5h, and then treating it with water vapor to obtain the depolymerization catalyst;

[0142] S2: under an inert gas atmosphere, continuously conveying the premix to a multi-stage series depolymerization reaction device for depolymerization to obtain a depolymerization product;

[0143] The multi-stage series depolymerization reaction device comprises at least three first depolymerization reactors, a second depolymerization reactor and a third depolymerization reactor which are sequentially arranged in series;

[0144] The operating parameters of the first depolymerization reactor include: temperature of 200°C to 240°C, pressure of 1.6MPa to 3.4MPa, and residence time of 10min to 90min;

[0145] The operating parameters of the second depolymerization reactor include: temperature of 260°C to 280°C, pressure of 2.4MPa to 3.6MPa, and residence time of 10min to 90min;

[0146] The operating parameters of the third depolymerization reactor include: temperature of 200°C to 240°C, pressure of 1.6MPa to 3.4MPa, and residence time of 20min to 300min;

[0147] S3: After the post-treatment reaction is completed, the depolymerization product is discharged from the outlet of the last stage depolymerization reactor, and the depolymerization product is purified, post-treated and detected and analyzed (the specific corresponding process is the same as step S3 in Example 1) to obtain the cleavage rate of the cyclic dimer.

[0148] The specific reaction parameters and cyclic dimer cleavage rates in Examples 32-36 are shown in Table 5.

[0149] Table 5 Specific reaction parameters and cyclic dimer cleavage rates in Examples 32-36

[0150]

[0151] Comparative Example 1

[0152] The only difference between this example and Example 1 is that the reaction conditions of the catalytic depolymerization in S2 are constant conditions, specifically: temperature of 260° C., pressure of 2.0 MPa, maintained for 60 min; the remaining steps and parameters are the same.

[0153] In this case, the cleavage rate of the cyclic dimer was 60.8%.

[0154] Comparative Example 2

[0155] The only difference between this example and Example 1 is that the reaction conditions of the catalytic depolymerization in S2 are constant conditions, specifically: temperature of 260° C., pressure of 2.0 MPa, maintained for 90 min; the remaining steps and parameters are the same.

[0156] In this case, the cleavage rate of the cyclic dimer was 61.4%.

[0157] In summary, the embodiments of the present application provide a method for catalytic depolymerization of nylon 6 extract oligomers. The present invention realizes the staged gradient depolymerization of nylon 6 extract oligomers by controlling the reaction pressure and reaction temperature in the depolymerization reaction system to change in stages with the reaction time. The depolymerization process is simple to operate and does not require the addition of other raw materials. The depolymerization rate of nylon 6 extract oligomers is improved, and the technical problems of poor catalytic depolymerization effect and complicated operation steps in the existing catalytic depolymerization process of nylon 6 extract oligomer concentrate are effectively solved.

[0158] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0159] In the present application, in the absence of any contrary description, the directional words used, such as "upper" and "lower", are specifically the directions of the drawings in the accompanying drawings. In addition, in the description of the present specification, the terms "including", "comprising", etc. refer to "including but not limited to". In this article, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply that there is any such actual relationship or order between these entities or operations. In this article, "and / or" describes the association relationship of the associated objects, indicating that there may be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A, B can be singular or plural. In this article, "at least one" refers to one or more, and "plural" refers to two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c", or "at least one of a, b and c", can both mean: a, b, c, ab (i.e. a and b), ac, bc or abc, where a, b, c can be single or plural, respectively.

[0160] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A method for catalytic depolymerization of nylon 6 oligomers, characterized in that: The catalytic depolymerization method comprises the following steps: The nylon 6 oligomer extract concentrate and the depolymerization catalyst are mixed, heated and ultrasonically treated to obtain a premix; Under an inert gas atmosphere, the premix is ​​subjected to a stepwise gradient depolymerization to obtain a depolymerization product; Purifying the depolymerization product to obtain caprolactam; The specific process of the step-by-step gradient depolymerization includes: the reaction pressure and the reaction temperature in the depolymerization reaction system are both subjected to step-by-step gradient changes along with the reaction time until the catalytic depolymerization reaction of the nylon 6 extraction oligomer is completed.

2. The catalytic depolymerization method according to claim 1, characterized in that: The weight percentage of water in the nylon 6 oligomer extract concentrate is 5wt% to 150wt%.

3. The catalytic depolymerization method according to claim 2, characterized in that: Based on the weight of the nylon 6 extracted oligomers, the added amount of the depolymerization catalyst accounts for 0.1wt% to 5wt% of the weight of the nylon 6 extracted oligomers.

4. The catalytic depolymerization method according to claim 3, characterized in that: The depolymerization catalyst is a solid acid catalyst or a liquid acid catalyst, and the solid acid catalyst includes at least one of a solid phosphoric acid catalyst, a solid sulfuric acid catalyst and a solid superacid catalyst.

5. The catalytic depolymerization method according to claim 4, characterized in that: The preparation method of the solid phosphoric acid catalyst comprises: The solid phosphoric acid catalyst is obtained by mixing pyrophosphoric acid and phosphorus pentoxide in a weight ratio of 1:(1.5-3.0), and then adding diatomaceous earth for heat treatment.

6. The catalytic depolymerization method according to any one of claims 1 to 5, characterized in that: The operation mode of the step-wise gradient depolymerization includes an intermittent catalytic depolymerization operation mode or a continuous catalytic depolymerization operation mode.

7. The catalytic depolymerization method according to claim 6, characterized in that: When the stepwise gradient depolymerization is performed by adopting an intermittent catalytic depolymerization operation mode, the step of performing stepwise gradient depolymerization on the premix under an inert gas atmosphere to obtain a depolymerization product comprises: Under an inert gas atmosphere, the premix is ​​added into a depolymerization reaction device at one time, and then a first depolymerization reaction stage, a second depolymerization reaction stage, a third depolymerization reaction stage and a fourth depolymerization reaction stage are sequentially performed to obtain a depolymerization product; The working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C to 220°C and the reaction pressure to 1.6MPa to 3.0MPa within 10min to 60min, and then maintaining for 10min to 60min; The working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C to 250°C and the reaction pressure to 2.0MPa to 3.4MPa within 10min to 30min, and then maintaining for 10min to 60min; The working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260°C to 280°C and the reaction pressure to 2.4MPa to 3.6MPa within 10min to 30min, and then maintaining for 30min to 120min; The working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. to 220° C. and the reaction pressure to 1.6 MPa to 3.0 MPa within 10 to 45 minutes, and then maintaining them for 30 to 300 minutes.

8. The catalytic depolymerization method according to claim 7, characterized in that: The working parameters of the first depolymerization reaction stage include: adjusting the reaction temperature to 200°C to 210°C and the reaction pressure to 1.6MPa to 2.6MPa within 10min to 40min, and then maintaining for 10min to 30min; The working parameters of the second depolymerization reaction stage include: adjusting the reaction temperature to 230°C to 240°C and the reaction pressure to 2.0MPa to 2.8MPa within 10min to 20min, and then maintaining for 10min to 30min; The working parameters of the third depolymerization reaction stage include: adjusting the reaction temperature to 260°C to 270°C and the reaction pressure to 2.4MPa to 3.0MPa within 10min to 20min, and then maintaining for 30min to 60min; The working parameters of the fourth depolymerization reaction stage include: adjusting the reaction temperature to 200° C. to 210° C. and the reaction pressure to 1.6 MPa to 2.6 MPa within 10 to 20 minutes, and then maintaining them for 30 to 150 minutes.

9. The catalytic depolymerization method according to claim 6, characterized in that: When the stepwise gradient depolymerization is performed in a continuous catalytic depolymerization operation mode, the step of performing stepwise gradient depolymerization on the premix under an inert gas atmosphere to obtain a depolymerization product comprises: Under an inert gas atmosphere, continuously conveying the premix to a multi-stage series depolymerization reaction device for depolymerization to obtain a depolymerization product; The multi-stage series depolymerization reaction device comprises at least three first depolymerization reactors, a second depolymerization reactor and a third depolymerization reactor which are sequentially arranged in series; The operating parameters of the first depolymerization reactor include: temperature of 200°C to 240°C, pressure of 1.6MPa to 3.4MPa, and residence time of 10min to 90min; The operating parameters of the second depolymerization reactor include: temperature of 260°C to 280°C, pressure of 2.4MPa to 3.6MPa, and residence time of 10min to 90min; The operating parameters of the third depolymerization reactor include: temperature of 200° C. to 240° C., pressure of 1.6 MPa to 3.4 MPa, and residence time of 20 min to 300 min.

10. The catalytic depolymerization method according to claim 9, characterized in that: The operating parameters of the first depolymerization reactor include: temperature of 230°C to 240°C, pressure of 2.0MPa to 3.0MPa, and residence time of 30min to 60min; The operating parameters of the second depolymerization reactor include: temperature of 265°C to 275°C, pressure of 3.0MPa to 3.2MPa, and residence time of 50min to 80min; The operating parameters of the third depolymerization reactor include: temperature of 210° C. to 230° C., pressure of 2.0 MPa to 3.0 MPa, and residence time of 50 min to 200 min.

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